1. Field of the Invention
The present invention relates to a semiconductor substrate, and more specifically, to a semiconductor substrate suitable to a polarizing element.
2. Related Background Art
In recent years, a study concerning quantum dots that is an ultrafine low-dimensional structure with a metal or semiconductor is carried out earnestly. Such a study has an object to realize a device having an excellent performance as is not found hitherto by using a quantum effect to be exhibited when electrons or holes are confined in a three-dimensional space. It is focused more and more with rapid development of nanotechnology in recent years. As a three-dimensional quantum confining structure, a concept of the “quantum dots” was first disclosed by Document 1: Y. Arakawa and H. Sakaki, “Multidimensional quantum well laser and temperature dependence of its threshold current”, Appl. Phys. Lett., Vol. 40, 1982, pp. 939. However, under the study circumstances at the very proposed time, the concept was not beyond the field of theoretical prediction. Thus, it was considered that the achievement of such devices was difficult. However, the quantum dots can be currently formed by several methods such as a microfabrication employing EB exposure and X-rays exposure, a selective growth onto a processed substrate, and self-assembly based on a three-dimensional island-shape growth in a molecular beam epitaxial (MBE) method owing to remarkable technological innovation concerning fine structures.
Here, in a method of forming quantum dots by a microfabrication using an exposure technology among the above specified methods, there is a problem such that a condition at an after-process growth boundary surface is poor since it is difficult to avoid process damage. Since this process damage may cause deterioration of characteristics, it is difficult to evaluate a variety of quantum effect characteristics with excellent precision in a substrate that is subjected to much process damage. In addition, there is a problem such that process steps are extremely complicated in a method of forming quantum dots by selective growth, which requires plenty of time and cost upon fabrication of the quantum dots.
For this reason, a method of forming quantum dots by self-assembly employing the MBE method is now becoming a main stream. This is because quantum dots of high quality, almost free from process damage, can be formed relatively easily by use of this method. This self-assembly technique is disclosed in Document 2: Y. Sugiyama et al. “Stacked InAs Self-Assembled Quantum Dots on (001) GaAs Grown by Molecular Beam Epitaxy”, Jpn. J. Appl. Phys., Vol. 35, 1996, pp. 1320, for example. That is, InAs quantum dots are formed on a GaAs substrate by using a three-dimensional island-shape growth mechanism caused by lattice mismatch (approximately 7.1%) between the GaAs substrate and InAs. It is directly confirmed by observations using an atomic force microscope and so on that the configuration of the quantum dots is formed in a semisphere lens shape of about several tens nanometer in diameter. The area density of the quantum dots generated on the substrate at random is approximately 1010 cm−2.
On the other hand, ring-shaped InAs quantum dots self-assembled on a GaAs substrate is disclosed in Document 3: A. Lorke et al, “Spectroscopy of Nanoscopic Semiconductor Rings”, Phys. Rev. Lett., Vol. 84, 2000, pp. 2223. In accordance with Document 3, InAs quantum dots are first formed on a GaAs substrate by a conventional method. The quantum dots are subjected to a thermal treatment after covered with a thin GaAs layer having the same height as that of the dots, and thereby the quantum dots are formed in a caldera-shape such that the top of the dots is caved in, which is caused by diffusion of InAs.